A fluorescent lamp operates by passing an electric discharge through mercury vapor contained within an envelope to produce short-wave ultraviolet (UV) light (generally at wavelengths of about 253.7 nm and 185 nm). The envelope bears a phosphor material which is caused to luminesce by the UV light, thereby emitting visible light. As a practical matter, many commercial fluorescent lamps may suffer from a decrease of lumen as a function of burning time. One reason for lumen decrease is the bombardment of the phosphor material by mercury ions and by 185 nm ultraviolet light from the discharge. The amount of mercury bound by the phosphor coating also increases with burning time, which may lead to a consumption of up to around half of the total amount of mercury consumed inside the lamp. This loss of mercury can also lead to lumen decrease. These effects may seriously limit the service life of the lamps.